From Stormwater Collection Systems Design Handbook

18.1 COMBINED SEWER OVERFLOW ( CSO)

Sewer systems that convey both sanitary sewerage and stormwater through a single pipe are referred to as combined sewer systems (CSSs). During dry weather a CSS is able to convey all flows to the waste water treatment plant. However, during periods of heavy rainfall or wet weather flow (WWF), the CSS capacity may be exceeded, resulting in a combination of untreated sewage and stormwater flowing in the same pipe(s), and may be discharged directly into nearby receiving water. These overflows are referred to as combined sewer overflows (CSOs). Manuals on CSO control have been published by the Water Environment Federation (WEF) (1989) and the U.S. Environmental Protection Agency (USEPA) (1993).

Retention storage facilities capture and dispose stormwater runoff through infiltration, percolation, and evaporation, whereas detention storage is temporary storage for stormwater runoff and CSO. In CSOs, temporarily stored flows are subsequently returned to the sewerage system at a reduced rate of flow, or the flow is discharged to a receiving water with or without treatment. The common elements of a typical CSS that are generally found in older cities are illustrated in Fig. 18.1.

Water and sewer utility services supply drinking water, remove sewage and wastewater, and/or install connections from processing facilities to private or public water lines and sewers. They supply water to commercial, industrial, and residential end-users; and receive sewage, wastewater, and other used water (such as gray water) for treatment

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Neil Armitage Department of Civil Engineering University of Cape Town Private Bag Rondebosch, 7701, South Africa 19.1 INTRODUCTION
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